An axis is a name for a continuum of possibilities between two extreme positions: so the axis between black and white is a scale of greys.

I can illustrate this idea by applying it to the description of haircuts. Rather than only being able to say of someone’s haircut that it is, for example, masculine or feminine, we’re as likely to want to say that it’s quite masculine, or quite feminine, or unisexual — somewhere in the middle.